In recent years, the research and development of high density optical disc systems to/from which information is recorded and/or reproduced by applying blue violet semiconductor laser beams having a wavelength of around 400 nm has been rapidly progressed. As an example, an optical disc having specifications, in which an objective lens having NA (Numerical Aperture) of 0.85, and a light source having wavelength of 405 nm are applied to record and/or reproduce information, can store information of 20-30 GB per a side. In this specification, it is called “high density DVD” from now on, which has the same size of DVD having a diameter of 12 cm (NA: 0.6, light source wavelength: 650 nm, memory capacity: 4.7 GB).
It is not sufficient as an optical disc player/recorder to be simply able to record and/or reproduce information onto/from the high density DVD. Taking account that DVDs and CDs (compact discs) on which various kinds of information is recorded are on a marketplace, it is not enough as value of the optical pickup apparatus to be simply able to appropriately record/reproduce information onto/from a high density DVD. It is necessary for an optical pickup apparatus to record and/or reproduce information not only onto/from a high density DVD but also a conventional DVD and a CD which users posses in order to raise the product value as an optical pickup apparatus having recording/reproducing compatibilities. From these backgrounds, it is required that an converging optical system for an optical pickup apparatus has not only a simple and low cost structure but also secure a predetermined light beam amount for forming an optical spot to appropriately record and/or reproduce information onto/from high-density DVDs, convention DVDs and CDs. An example of optical pickup apparatus having compatibilities is described in Japanese Patent Application Open to Public No. JP2002-298422.
However, according to JP2002-298422 and JP2001-60336, a single objective lens (a lens group) is designed to converge respective three waves onto respective optical discs with minimum aberration (triple compatibility system). As a result, even though the objective lens is preferable from the cost point of view, the design margin for respective three wavelengths is small, and there is a problem that, in general, the design of the objective lens is difficult. Particularly, there is a problem that the objective lens having two-lens structure disclosed in JP2001-60336 based on the triple compatibility system requires the highly precise adjustment of an optical axis of those lenses. There is another problem that the thickness of the objective lens in the optical axis direction becomes thick. Accordingly, it is understood that there is a problem that to achieve the triple compatibility system by a single objective lens is unfavorable from the viewpoints of mass-productivity and the size of the objective lens.
On the contrary, for example, an optical pickup apparatus separately having two objective lenses, one for high density DVD and the other for DVD and CD, which can be selectively applied to converge recording beams and reflected beams according to the kind of optical disc to realize the triple compatibility system has been developed. However, in general, since the number of positions where the optical disc is placed is one, it is necessary to change the objective lens to be applied according to the kind of the optical disc. Since there are two independent objective lenses, it is necessary to conduct the coma adjustment for the tilt of each objective lens to appropriately record and/or reproduce information onto/from the optical disc. For example, it will be possible to correct the affect at a certain degree, even though the shift and the tilt of the objective lens occur when the optical pickup apparatus is assembled, if the optical pickup has a relative tilt angle changing mechanism, which can change the relative tilt angle formed between the optical disc and the objective lens. However, since the load of the relative tilt angle changing mechanism increases as the tilt angle becomes large, it is preferable that the tilt angle is minimized.
Here, it will be also possible to suppress the coma aberrations of both objective lenses by adjusting the tilt angle of an actuator base against the optical disc according to the tilt angle of the optical disc by attaching the two objective lenses having a small coma aberration on the actuator via a lens holder. However, according to this method, the coma aberration of the objective lens itself has to be small. In this case, there is a problem that it is difficult to manufacture such an objective lens. Further, when adhering the two objective lenses onto the actuator via the lens holder, it may be possible to adjust the angle formed between the objective lens and the optical pickup so that the respective optical axes of the two objective lenses become orthogonal to the optical disc. However, according to this method, there is a problem that the difference of respective coma aberrations of respective objective lenses cannot be fully adjusted. Accordingly, it is necessary to put priority on one of the objectives lenses or to adjust the coma aberration into the middle point between two objective lenses. Further, there is anther problem that the coma aberration caused by the position shift of three different light sources having different wavelengths cannot be corrected.